I also am not an IFA, but having retired early 13 years ago, after nearly 40 years of international banking experience, I would like to make the following observations:
Unless the children are receiving a very substantial income, tax should be nowhere near 50%, especially if the funds are held offshore. The usual reason for overseas trusts was to reduce the liability to UK taxation to nil if possible.
If the Trustees chose an inappropriate investment vehicle, I would raise the question of their possible liability for damages. If you cannot do this, then the beneficiaries should do so!
I presume that the Trustees are 'professional' and are charging fees accordingly. These should be calculated as a % of the total fund and charged annually pro rata to each beneficiary. Therefore, should one of the beneficiaries withdraw their share, their proportion of the charges would have been charged to their settlement, leaving the same ratio of outstanding charges as the remaining shares in the funds.
I commend you eldest for not wanting to withdraw his share of the fund at the present time, but my inclination would be to move the funds elsewhere if the current Trustees are so inefficient (unless any Capital Gain they have managed to realise over the last 4 years substantially outweigh the tax and charges deducted). The wording of the Trust Deed could possibly prevent him continuing to use the Trust as an investment due to his age. Furthermore, as you son is over the age of maturity of his share of the legacy, he has every right to demand a copy of the Trust Deed, which he could share with you to ascertain the Terms and Conditions of the Trust.